1. Field of the Invention
The present invention relates to a particulate porous ammoxidation catalyst which can be advantageously used in producing acrylonitrile or methacrylonitrile by reacting propylene, isobutene or tert-butyl alcohol with molecular oxygen and ammonia in a fluidized-bed reactor. More particularly, the present invention is concerned with a particulate porous ammoxidation catalyst which comprises a metal oxide and a silica carrier having supported thereon the metal oxide, wherein the metal oxide contains at least two elements selected from the group consisting of molybdenum, bismuth, iron, vanadium, antimony, tellurium and niobium, the catalyst having a particle diameter distribution wherein the amount of catalyst particles having a particle diameter of from 5 to 200 μm is from 90 to 100% by weight, based on the weight of the catalyst, and having a pore distribution wherein the cumulative pore volume of pores having a pore diameter of 80 Å or less is not more than 20%, based on the total pore volume of the catalyst, and the cumulative pore volume of pores having a pore diameter of 1,000 Å or more is not more than 20%, based on the total pore volume of the catalyst. The present invention is also concerned with a method for efficiently producing this catalyst. The ammoxidation catalyst of the present invention exhibits not only high activity in producing the desired product but also high attrition resistance when used on a commercial scale. Therefore, the catalyst of the present invention is advantageous in that, when the catalyst of the present invention is used for performing a catalytic ammoxidation of propylene, isobutene or tert-butyl alcohol in a fluidized-bed reactor, acrylonitrile or methacrylonitrile can be produced stably in high yield.
2. Prior Art
It has been well known to produce acrylonitrile or methacrylonitrile by ammoxidation of propylene, isobutene or tert-butyl alcohol, namely, a reaction of propylene, isobutene or tert-butyl alcohol with molecular oxygen and ammonia. A number of proposals have been made with respect to catalysts for use in the ammoxidation of propylene, isobutene or tert-butyl alcohol. For example, Examined Japanese Patent Application Publication No. Sho 38-17967 proposes an oxide catalyst containing molybdenum, bismuth and iron, and Examined Japanese Patent Application Publication No. Sho 38-9111 proposes an oxide catalyst containing antimony and iron. Further, various improvements have been proposed with respect to these ammoxidation catalysts.
A number of proposals have been made to improve an ammoxidation catalyst by changing the composition thereof, and such proposals have contributed to the improvement of the catalyst performance. However, there have not been made many proposals to improve an ammoxidation catalyst by changing the physical structure of the catalyst. As examples of prior art documents disclosing such proposals, there can be mentioned Examined Japanese Patent Application Publication No. Sho 57-56373 (corresponding to U.S. Pat. No. 4,264,476) which discloses a catalyst for ammoxidation of propylene, containing molybdenum, bismuth, iron, cobalt and zirconium as essential components, and Unexamined Japanese Patent Application Laid-Open Specification No. Sho 57-75147 which discloses a catalyst for ammoxidation of propylene, containing molybdenum, bismuth and antimony as essential components, wherein, in each of these catalysts, the amount of silica carrier, average pore diameter, total pore volume and specific surface area are, respectively, in specific ranges.
Each of the above-mentioned two patent documents (in which it is attempted to improve the catalyst performance by changing the physical structure of the catalyst) discloses a catalyst (having a specific physical structure) for use in producing acrylonitrile in a fluidized-bed reactor. However, there are no disclosures about the pore distributions of the catalysts in these two patent documents.
On the other hand, proposals paying attention to the pore distribution of an oxide catalyst, have been made in the following patent documents. Unexamined Japanese Patent Application Laid-Open Specification No. Sho 57-119837 discloses a catalyst for use in the oxidation of an olefin in a fixed-bed reactor, wherein the catalyst has an average pore radius of 2,000 Å or more. Unexamined Japanese Patent Application Laid-Open Specification No. Sho 58-113141 (corresponding to GB 2030885A) discloses a process for producing methacrolein. Claim 3 of this patent document describes that the cumulative surface area of pores having a diameter smaller than 100 Å is not more than 3%, based on the surface area of the catalyst. International patent application publication No. WO03/039744 discloses a metal oxide catalyst for use in the commercial production of acrolein or acrylic acid by oxidation of propylene, wherein the physical structure (including the pore distribution) of the catalyst is specified in detail.
However, none of these oxide catalysts can be used in a fluidized-bed reactor. Specifically, in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Sho 57-119837, claim 1 describes that the catalyst is for use in a fixed-bed reactor. Further, the catalyst of this patent document has an average pore radius as large as 2,000 Å or more and, hence, it is presumed that the catalyst exhibits poor mechanical strength. Also, the catalyst of this patent document is an extrusion-molded product (e.g., a cylinder-shaped article having a diameter of 4 mm and a length of from 4 to 8 mm in the working examples) and, hence, it is presumed that the catalyst exhibits poor fluidity. Therefore, it is apparent that the catalyst disclosed in this patent document cannot be used in a fluidized-bed reactor. The above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Sho 58-113141 discloses a process for producing methac-rolein, wherein the mode of reaction is not specified. However, in the working examples of this patent document, there is a description that the catalyst is obtained as a pellet having a diameter of 4.8 mm and, hence, it is apparent that the catalyst proposed in this patent document exhibits poor fluidity and is for use in a fixed-bed reactor. Therefore, this catalyst cannot be used in a fluidized-bed reactor. In the above-mentioned international patent application publication No. WO03/039744, the mode of reaction is not specified; however, the catalyst proposed in this patent document has a pore distribution wherein the cumulative pore volume of pores having a pore diameter of from 0.1 to 1 μm is as large as not less than 20%, based on the total pore volume of the catalyst. That is, the ratio of pores having a large diameter is great in the pore distribution of this catalyst and, hence, it is presumed that this catalyst exhibits poor mechanical strength. Further, in the working examples of this patent document, there is a description that a particulate catalyst precursor is pelletized in the form of a tablet having a diameter of 5 mm and a height of 4 mm. Therefore, it is presumed that the catalyst exhibits poor fluidity and, hence, it is apparent that the catalyst proposed in this patent document is for use in a fixed-bed reactor and cannot be used in a fluidized-bed reactor.
Further, each of U.S. Pat. No. 3,397,153 and Examined Japanese Patent Application Publication No. Hei 2-47264 (corresponding to U.S. Pat. No. 4,590,173 and EP 0153077B) discloses a process for producing a sintered catalyst having a low density, which is prepared using a silica raw material which is a mixture of two silica sols of different average silica particle diameters. In any of these two patent documents, there is no disclosure about the data of the pore distribution of the catalyst. In the former of the two patent documents, it is intended to improve the economy of the catalyst by imparting a low density to the catalyst. In the latter of the two patent documents, it is intended to improve the mechanical strength of an oxide catalyst containing antimony, by imparting a low density to the catalyst. In these two patent documents, there are no descriptions suggesting that there is any relationship between the pore distribution of a catalyst and the yield of the desired product obtained using the catalyst, and there are no suggestions about an oxide catalyst exhibiting an improved yield of the desired product.
Any of the conventional catalysts described hereinabove is still unsatisfactory in respect of the yield of the desired product. Therefore, it has been desired to develop a catalyst which can be advantageously used in the ammoxidation of propylene, isobutene or tert-butyl alcohol in a fluidized-bed reactor, so that acrylonitrile or methacrylonitrile can be stably produced in high yield.